Methods of cell population distribution assisted read margining

ABSTRACT

A memory using techniques to extract the data content of its storage elements, when the distribution of stored states is degraded, is presented. If the distribution of stored states has degraded, secondary evaluations of the memory cells are performed using modified read conditions. Based upon the results of these supplemental evaluations, the memory device determines the read conditions at which to best decide the data stored.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. application Ser. No.11/535,853 filed Sep. 27, 2006, now U.S. Pat. No. 7,886,204, and is alsorelated to U.S. application Ser. No. 11/535,879, entitled “Memory withCell Population Distribution Assisted Read Margining,” by Carlos J.Gonzalez and Daniel C. Guterman, now U.S. Pat. No. 7,716,538, whichapplications are incorporated herein in their entirety by thisreference.

BACKGROUND OF THE INVENTION

The present invention relates generally to reading the data content ofnon-volatile and other memory devices, and, more particularly, to usinginformation on the distribution of program levels of a memory cellpopulations to more accurately read the content of degradeddistributions.

As flash and other memory devices migrate to smaller geometries, theinfluence of a number of phenomena that negatively impact the robustnessof data storage increases. Included in these factors areover-programming, read and program disturb, and data retention issues.These problems are often further aggravated as the number of states percell is increased and as the operating window of stored thresholdvoltages shrinks. These factors are generally accounted for in thedesign phase of the memory devices through various tradeoffs that can bemade within the design. These tradeoffs may increase or decrease theinfluence of one or the other of these factors, and/or tradeoff some ofthese factors against others, such as performance, endurance,reliability, and so on. In addition to tradeoffs within the memorydesign, there are a number of system-level mechanisms that may beincorporated to compensate for these phenomena, where needed, to achieveproduct-level specifications. These system mechanisms include ECC,wear-leveling, data refresh (or “Scrub”), and read margining (or “HeroicRecovery”), such as are discussed in U.S. Pat. Nos. 7,012,835, 6,151,246and, especially, 5,657,332.

The above phenomena generally have the impact of affecting thedistribution of cell voltage thresholds, either during programming,during subsequent memory operations, or over time, and they generallyhave a larger impact in multi-state memory storage relative to binarymemory storage. The impact is typically to spread the voltage thresholdlevels of a given memory state within a population of cells, and, insome cases, to shift cell threshold levels such that they read in anerroneous state under normal read conditions, in which case the databits for those cells become erroneous. As memories having smallergeometries become integrated into storage products, it is expected thatthe memory-level tradeoffs required to overcome the anticipated memoryphenomena will make it difficult to achieve the required product-levelspecifications. Consequently, improvements to these devices will berequired.

SUMMARY OF THE INVENTION

The present invention presents a memory device and methods ofdetermining its data content. The memory cells of the device areevaluated at a first reference condition and a plurality of secondaryreference conditions. Based on comparing the number of memory cellsevaluated at the first reference condition and the second referenceconditions, the memory device establishes a read condition for a datastate based on the rate of change of number of memory cells evaluated atthe plurality of reference conditions.

In some embodiments, the evaluations of the memory cells using aplurality of secondary read conditions is performed in response todetermining that an evaluation using standard read conditions has anunacceptable level of error. Information on the distribution ofprogrammed state populations of the memory cells is extracted based onthe results of the evaluations using the standard read conditions andthe plurality of secondary read conditions. Modified read conditions,which differ from the standard read conditions, are determined at whichto evaluate the memory cells to determine their data content based onthe information on the distribution of programmed state populations.

Additional aspects, advantages and features of the present invention areincluded in the following description of exemplary examples thereof. Allpatents, patent applications, articles, books, specifications, otherpublications, documents and items referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of a term between any of the incorporatedpublications, documents or things and the text of the present document,the definition or use of the term in the present document shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 shows an example of a degraded distribution of programmed memorystates.

FIG. 2 is a flowchart illustrating aspects of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention is related to reading the data content of memorysystems. When data, whether stored in either binary or multi-state permemory cell form, is programmed into a memory, the population ofindividual cells programmed to a given state will form distributionsaround the desired values of the parameter corresponding to therespective storage states. For example, in the case of a flash memory,its threshold voltage characterizes a particular data state. If a datastate corresponds to a threshold voltage of, say, 2 volts, the cellsprogrammed to this state will not all end up at exactly 2.0 volts, butrather be spread out over a distribution mostly above the correspondingprogram verify level for that state. Although at the time of programmingthe distributions corresponding to the data states may be well definedand clearly separated, over time and operating history the distributionsmay spread. This degradation can lead to misreading of data as the readconditions that are used to distinguish one state from another may nolonger correctly read the state of a cell whose threshold value hasshifted too far.

As discussed in the Background section, as size of memory devices becomeever smaller, use lower operating voltages, and store more states permemory cell, the influence of various phenomena that negatively impactthe robustness of data storage increases. These factors includeover-programming, read and program disturb, data pattern/historyinfluences and data retention issues associated with a given memorytechnology. These factors are generally accounted for in the designphase of the flash and other memory devices, with a number of tradeoffsmade within the design that may increase or decrease the influence ofone or the other and/or that tradeoff some of these factors againstothers, such as performance, endurance, reliability, and so on. Beyondthe tradeoffs inherent in a given memory design, there are a number ofsystem-level mechanisms that may be designed-in to compensate for thesephenomena where needed to achieve product-level specifications. Thesesystem mechanisms include error correction code (ECC), wear-leveling,data refresh (or “scrubbing”), and read margining (or “heroicrecovery”).

Such previous methods, along with appropriate structures, are describedin U.S. Pat. No. 5,657,332, which is fully incorporated herein andreferenced in many locations, can be considered as base embodiments ofcircuitry and other memory device elements upon which the variousaspects of the present invention can be incorporated. When reference toa specific memory array embodiment is needed, the exemplary embodimentof the memory can be taken as a NAND type flash memory such as thatdescribed in U.S. Pat. Nos. 5,570,315, 5,903,495, and 6,046,935.

The various phenomena affecting the distribution generally have theimpact of affecting the distribution of cells either during programming,during subsequent memory operations, or over time, and they generallyhave a larger impact in multi-state memory storage relative to binarymemory storage. The impact is typically to spread the threshold voltage(or other applicable state parameter) in a population of cells within agiven state, and in some cases, to shift cells' threshold voltage suchthat they read in an erroneous state under normal read conditions, inwhich case the data bits for those cells are erroneous.

A typical situation is illustrated schematically in FIG. 1. FIG. 1 showsthe distribution of memory storage units versus the parameter, V_(th),that defines differing memory state threshold voltage distributions,D_(A) and D_(B), for two corresponding data states, A and B,respectively. To make the example concrete, these can be taken as datastates for cells of a flash memory, where the parameter is thresholdvoltage. The distributions for two states, A and B, are shown. These maybe the only states in the case of a binary memory or two adjacent statesof a multi-state memory. When these states are initially programmed,their associated threshold voltage levels are based on a set of verifylevels, where all the cells of a given data state are programmed untiltheir threshold voltage levels lie above the corresponding verify level.These initial post-programming distributions for the A and B states areshown as D_(A) and D_(B) and used corresponding verify points V_(Aver)and V_(Bver). Programming of a given cell in this memory exampleproceeds from lower voltage threshold levels to higher levels, and isnormally terminated once it verifies successfully, so that thesuccessfully programmed cells of a given state will typically lie abovethe verify level, the maximum of which is usually dictated by the amountof movement that results from one given programming pulse. Because somecells program relatively quickly compared to the mainstream cellpopulation, and no provision is typically made for cells that programmedtoo quickly, this can lead to somewhat of a tail on the higher end ofthe threshold voltage distribution. Various programming techniques areknown to improve the tightness of the distributions, some of which aredescribed in U.S. Pat. Nos. 6,738,289, 6,621,742, and 6,522,580.

To read the data content of this memory, the verify points can then beused as read compare points, although typically a read point is shiftedsomewhat in the less programmed (lower voltage) direction to providesome safety margin. For instance, in FIG. 1 the point V_(Br0) can beused as the normal read point to distinguish A states from B states. (Inthe case of a multi-state memory, it will actually distinguish states Aand any lower states from state B and any higher states, whereintechniques for disentangling the various multi-states are familiar inthe art.) Due to the various mechanisms mentioned above, thedistributions D_(A) and D_(B) tend to degrade as shown schematically bythe distributions D′_(A) and D′_(B). By placing V_(Br0) somewhat lowerthan V_(Bver), some allowance for the degradation is made; however, iftoo many cells have drifted below V_(Br0), the capability of the ECCbecomes overwhelmed, and the system cannot successfully extract thecorresponding data content. V_(Br0) could of course be moved further tothe left, a still lower voltage level (“heroic recovery), but eventuallysuch shifted reads will result in too many cells that properly belong tothe A state to be misread as B state. Furthermore, following data write,some of the A states may possibly shift upward as the A distributiondegrades via mechanisms listed above, thereby further aggravating thesituation. (As discussed further below, although the discussion ispresented here in terms of varying the compare point, keeping thecompare point the same but changing the bias levels on a cell being readcan alternately achieve the same end.)

With the integration of higher density memories into storage products,it is anticipated that the memory-level tradeoffs required to overcomethe anticipated memory phenomena will make it more difficult still toachieve the required product-level specifications. One of thesystem-level mechanisms anticipated to provide a benefit to suchproducts is the following type of read margining during read retries,referred to as “heroic recovery”, which is employed upon detection of anuncorrectable ECC error under nominal read conditions. Heroic recoveryconsists of re-reading data during retries under shifted read biasconditions or shifted compare points, essentially changing thediscrimination points between states, in an attempt to recover cellsthat read in the erroneous state under normal conditions to their properstate. Heroic Recovery has a few drawbacks that need to be overcome inorder to provide the best benefit to the product. Because the storagesystem relies on ECC to detect erroneous bits, and because there is noindependent indication of which direction cells may have shifted (suchas a count of cells expected in each state), there is no way for thesystem to know the actual direction that the cells in erroneous stateshave in fact shifted. The bias conditions generally follow apre-determined sequence, designed based on the expected influence of theshifting phenomena, which may be toward either the more programmed ormore erased states. The actual direction of the shift experienced by thecells may be counter to expectations due to the fact that there arenumerous independent influences. In the absence of safeguards, it ispossible that the biasing of the read conditions may cause a largeenough number of cells to be read in erroneous states so as to overwhelmthe ECC capabilities. Once overwhelmed, the ECC algorithm may eitherfail to detect an ECC error (misdetection), or to erroneously “correct”the set of data bits (miscorrection), in either case leading toerroneous data being passed as good data.

Various approaches can be used to improve the robustness of the heroicrecovery mechanism. One of these is the use of reference or trackingcells, such as are described in U.S. Pat. Nos. 5,172,338, 6,222,762 and6,538,922. Under this arrangement, a number of cells are programmed toknown (i.e. reference) states. During read retries, these cells can beread to a fine granularity, and their distribution used to estimate themain cell population. In this way excessive shifts from nominal aredetected, information from which is then used to guide the heroicrecovery bias conditions. This method has the drawback of requiringadditional cells, which adds cost to each flash memory die.Additionally, because in practice the tracking cell population is muchsmaller than the main population, their statistics may not reflect thepopulation shifts with sufficient accuracy. Nevertheless, it should benoted that tracking cells can be utilized in conjunction with thepresent invention for the advantages they provide.

Another approach is to minimize the likelihood of failure. For example,the sequence of bias conditions and ECC correction capabilities utilizedduring each iteration of read retries can be designed such that it willminimize the likelihood of ECC misdetection or miscorrection. Thismethod may lead to long retry sequences, however, since typically thesystem tries the safest combinations first, and attempts the morepowerful combinations that carry the most risk only after exhausting theearlier, safer retries. This is often not a robust solution, and it isbest used in conjunction with a safeguard.

According to one aspect of the present invention, the storage systemuses knowledge of the main cell population itself as a safeguard toavoid heroic recovery retries from biasing reads in the wrong direction.In a basic embodiment, the implementation relies on the fact that theexpected disturb mechanism to be overcome will more frequently shiftcells toward the more erased states, and hence the heroic recovery biaswill always be in the direction of the more erased states. Upondetecting uncorrectable ECC error during nominal read, the system willperform a number of reads under biased conditions in small biasincrements in the direction of the erased states, and count the numberof cells in each state at each step. The system will then compare thenumber of cells that change states and determine the gradient or rate ofchange with each step. If it is determined that the rate of cellsshifting from one population to the next increases with each step, thenthe discrimination point will be understood to be penetrating a cellpopulation (e.g. penetrating population A in FIG. 1 when V_(Br) isshifted too far negatively), in which case the system will not invokeHeroic Recovery.

As an additional safeguard, the system could perform a number of readsunder biased conditions in the direction of the programmed states, andif it is determined that the rate of cells shifting from one populationto the next is decreasing, the system would not invoke heroic recovery.Heroic recovery would only be invoked when all cell count-basedconditions indicate it to be appropriate. An extension of this idea isto use the rate of change of cell populations to guide or limit theamount of bias during Heroic Recovery.

These concepts can be illustrated by returning FIG. 1. The degradeddistributions of states A and B are shown schematically as the brokenlines of distributions D′_(A) and D′_(B), and show significantspreading, particularly towards a less programmed (lower thresholdvoltage) condition. The goal is to determine the bias conditions orcompare points at which to optimally read the B state with minimum riskof exacerbating existing error. The main discussion is given in terms ofvarying a voltage for the compare point for simplicity in illustration,in which case the question comes down to deciding what is the bestcompare voltage to be used to extract data.

As shown in FIG. 1, a fair amount of the D′_(B) has shifted belowV_(Br0), the nominal read bias condition. If the number in error is notso great as to overwhelm the ECC, the data can be extracted based onthis standard read. If the normal read is not successful, heroicmeasures can be taken. A number of secondary read points, in thisexample the three levels V_(Br1), V_(Br2), V_(Br3), at progressivelylower voltages are shown, associated with the heroic reads. Each ofthese will progressively correctly detect more of the B state cells thathave shifted to lower voltages. However, beyond a certain point, theseoffset reads will begin to pick up outliers at the top end of the Astate distribution. As shown in the figure, at V_(Br2) the lowered readpoint is still largely confined to the bottom part of D′_(B), whereas byV_(Br3) it has begun to penetrate D′_(A). (As shown in the detail, thenumber of states counted will be (D′_(A)+D′_(B)), which begins to have anon-negligible contribution between V_(Br3) and V_(Br2).) Consequently,in this example the optimal read point is probably a little belowV_(Br2), but closer to V_(Br2) than V_(Br3). The present invention usesthese different read points to determine the characteristics of thedistribution and, according to various embodiments, to, in turn,determine which of these secondary read points is the best choice toeither extract data or to establish a new read point at which to readthe data content. In FIG. 1, the best choice of the secondary readpoints would be V_(Br2), while in an embodiment that extrapolates orinterpolates an optimal (to a required accuracy) read point, this wouldlie somewhat to the left of V_(Br2).

Let N₀ be the number of states lying above V_(Br0), N₁ be the number ofstates lying above V_(Br1), N₂ be the number of states lying aboveV_(Br2), and N₃ be the number of states lying above V_(Br3). (Again, thenumber of secondary read points can vary according to the embodiment.)Note that the data content need not actually be extracted in these reads(and, if there is too much error, this may not even be possible), butonly that the number of states lying above the read point need bedetermined. As exemplified in FIG. 1, each of these numbers becomesprogressively largely; but the magnitude by which each of theseincreases (relative to the change in read parameter) becomes less asthey move further into the tail of the distribution—at least until theybegin to penetrate the upper end of the next lower state distribution.(Note that if the read points are not evenly spaced, this is preferablycompensated for.) Consequently, the important quantity is the differencebetween the N values.

Calling the difference between N values Δ, this gives(N ₁ −N ₀)=Δ_(1,0),with Δ_(2,1) and Δ_(3,2) similarly defined. Although the various Ns willpick up not just the cells in the B distribution but also any higherstates, these higher states will not contribute to Δ_(1,0), since theircontribution remains the same within each of the N values, and thereforewill cancel out. Also, there is no need for an actual read of the datacontent or evaluation of ECC, since, at this point, the process is justtrying to find the best (or sufficiently good) read point at which toperform this data extraction. In the example of FIG. 1, Δ_(1,0) will belarger than Δ_(2,1), so that a read point between V_(Br2) and V_(Br1)will be better than a read point between V_(Br1) and V_(Br0). How, withΔ_(3,2) slightly larger than Δ_(2,1), V_(Br3) is likely to have begunencroaching upon the A distribution. Consequently, V_(Br2) can used asthe read point for data extraction or the values of Δ_(3,2) and Δ_(2,1)could be analyzed to determine a yet more optimal value. In onevariation, additional reads of the region between V_(Br2) and V_(Br3)can be performed to refine the process. However, it is not necessary tofind the best point, but merely one for which the data content can beextracted correctly. Consequently, the selected read point need not bethe optimal point, but simply one of these same set of read points, asdescribed above, which offers the best (lowest) value for Δ. Forexample, the point V_(Br2), is probably the best choice in FIG. 1 andcan used to extract the data content. Alternatively, even though Δ_(1,0)is greater than Δ_(2,1) and, consequently, V_(Br2) is better (in thesense correctly reading more cells) than V_(Br1), if Δ_(1,0) is smallenough (such as less than a bound that could, for example, be a settableparameter), V_(Br1) could be selected for extracting the data.

Although the discussion here is in the context of find a read point toextract the data content, it can also be used to improve various datarefresh or scrub methods, such as those found in U.S. Pat. No.5,657,332, whose functions are not primarily to provide data for someexternal (end user/use) application, but rather to provided internalhousekeeping functions, confined within the memory device, itself.

The discussion of the process thus far has been described mainly interms of varying a compare or reference voltage to which the state ofthe memory cell is compared, since this is perhaps the easiest contextin which to describe the invention with respect to FIG. 1. However, asis known in the art, keeping the read reference values the same andchanging the bias on the cell being read can also accomplish this end,either independently of or used in conjunction with varying thereference point. In EEPROM and other charge storing transistor basedmemory technologies, this changing cell bias is typically done byvarying the control gate voltage of the memory cell, although the levelon the source, drain, or substrate (or even other transistors within,for example, the NAND string of the cell) can be varied as well. By wayof example, the varying of reference levels as opposed to varying biasconditions is discussed with respect FIG. 6 b as opposed to 6 a of U.S.Pat. No. 5,657,332, where the reference parameter (or parameters) arecurrent. Similarly, although the discussion of FIG. 1 was based on avoltage comparison, other parameters indicative of a cell's programminglevel (voltage, current, time, or frequency) can be used, as discussedin the various references explicitly cited herein. Furthermore, therequired voltages, currents, and so on needed for bias levels, referencelevels, or both can be generated by the various known techniques(reference cells, band gap based generators, etc.).

Further, the present techniques are not limited to only flash memories.A number of memories exhibit the characteristics described with respectto FIG. 1, such as the various non-volatile memory devices described inU.S. patent publication US-2005-0251617-A1; consequently, the variousaspects of the present invention has great utility for any of thosetechnologies for which the distribution of programmed states has atendency to degrade. It can also be applied to volatile memories thatsuffer from this sort of degradation due to leakage or other data draft(such as in a DRAM where there may be capacitor leakage) similar to thatdescribed with respect to FIG. 1. Also, as described above, althoughFIG. 1 shows only two states, the present invention is applicable notonly to binary (where A and B are the only states) but also tomulti-state memories (where A and B represent two adjacent states of amulti-state memory).

In a typical embodiment of a memory device having a controller portionand a memory portion, this process would in most cases be managed viathe controller, in a firmware implementation. In other embodiments itcan be executed on the memory itself, should that memory unit havesufficient capability, or it can be distributed between the controllerand memory portions. In still other embodiments, such as within memorycards lacking a full controller (e.g. xD cards or MemoryStick), some orall parts of the process can be managed by the host. For any of thesevariations, the different portions of the process can be implemented inhardware, software, firmware, or a combination of these.

FIG. 2 is a flowchart to illustrate some of the various aspects of thepresent invention. The process begins at step 201 when a standard readprocess, using the usual bias conditions and reference values, isperformed. At step 203, it is determined whether the data content issuccessfully extracted from the memory cells. If the read is successful(Yes out of step 203), the data stored in the cells is sent out (205).For example, the memory may have some amount of error, but within thelimits of the corresponding error correction code, in which case thedata content can still be extracted. (If there is some amount of error,but the content can still be extracted, a scrub operation can optionallybe performed.)

Should the read not be successful, for example returning an ECCuncorrectable error signal rather than the data, the process goes to themain aspects of the invention, beginning with step 207. In someembodiments, the process can jump directly from step 207 (eliminatingtest condition 203), where the preferred read conditions are determinedas part of a standard sensing operation, or the invocation of theprocess beginning at step 207 may be due to other reasons than thedetermination at step 203, such as if a certain amount of time haselapsed since the last read or a large numbers of possibly disturbingoperations have been previously executed. At step 207, the first of thesecondary read conditions are established. These can differ from thenormal read in a number of ways, which can be used individually or incombination. One of these is to shift the value of the read comparisonparameter, such as the voltage, current, time, or other parameter valueindicative of the state. (This is similar to what is shown in FIG. 6 bof U.S. Pat. No. 5,657,332 for a current based comparison.) Another isto change the bias conditions on the cells being read. For the exemplaryflash memory embodiment and other charge storing transistor embodiments,this is typically done by changing the control gate voltage applied tothe cells (as in FIG. 6 a of U.S. Pat. No. 5,657,332), although this canalso be done using changes to the source/drain voltage levels, othergate levels in a NAND string, or other bias shifts instead of (or inaddition to) altering the control gate level.

The secondary read is executed at step 209. In more basicimplementations of Heroic Recovery, the data can be output at this pointif the secondary read is successful. As noted above, this evaluationneed not be a read in the full sense of extracting data, but only needcount the number of cells that register above the compare point.

Some of the primary aspects of the present invention are found in steps211, 213, and 215. At step 211, the change in the number of states readis determined at 211. This will compare, for example, the differencebetween the number of cells above a normal read parameter and the numberof cells above a first secondary read parameter with the differencebetween the number of cells above the first secondary read parameter andthe number of cells above a second secondary read parameter. Asdescribed above, this is done to determine characteristics of thedistribution. For example, if only a few additional cells are picked upin going from the normal read to the first secondary read, but moreadditional cells are picked up in going from the first secondary read toa second secondary read, the read point or bias shift of the secondsecondary read has likely go too far and is penetrating into thedistribution of the next data state.

At step 213 it is determined whether more secondary reads are to beexecuted. The number of secondary reads can either be a fixed value (forexample, as a settable parameter) or can be determined based upon theresults of the earlier reads. In the fixed value example, a parameterkeeping track of the supplemental reads would be incremented at eachiteration and step 213 would decide whether it has reached its limit. Inembodiments using earlier evaluations, 213 could, for example, determinewhether Δ has begun to increase. Even in embodiments that decide step213 based on earlier reads, it may be useful to keep track of the numberof iterations and set a maximum number of these. If more reads are to beexecuted, the flow loops back to step 207; if not, it goes to step 215.

In step 215, the read conditions at which the data will be extracted aredetermined. This may be one of the reads performed at step 209 or anadditional read, in which case the additional read is executed at step217. In either case, the data stored in the cells is sent out (205).

Therefore, the present examples are to be considered as illustrative andnot restrictive, and the invention is not to be limited to the detailsgiven herein, but may be modified within the scope of the appendedclaims.

The invention claimed is:
 1. A method of determining the data content ofa memory device having a plurality of memory cells each storing one ofat least two data states, comprising: evaluating the memory cells usingstandard read conditions; determining whether the memory cells evaluatedusing standard read conditions have an unacceptable level of error; inresponse to determining that the memory cells evaluated using standardread conditions have an unacceptable level of error, performingevaluations of the memory cells using a plurality of auxiliary readconditions; extracting information on the distribution of thresholdvalues of the memory cells based on said evaluating the memory cellsusing the standard read conditions and the evaluations of the memorycells using the plurality of auxiliary read conditions; and determiningmodified read conditions differing from the standard read conditions atwhich to evaluate the memory cells to determine their data content basedon said information on the distribution of threshold values.
 2. Themethod of claim 1, wherein said extracting information on thedistribution of threshold values of the memory cells includes comparingthe number of memory cells evaluated using the standard read conditions,the number of memory cells evaluated using the plurality of auxiliaryread conditions, and the rate of change of the number of memory cellsevaluated at the standard and auxiliary read conditions.
 3. The methodof claim 1, wherein said determining whether the memory cells evaluatedusing standard read conditions have an unacceptable level of error isbased on an error correction code result.
 4. The method of claim 1,wherein the modified read conditions are one of said auxiliary readconditions.
 5. The method of claim 1, wherein the modified readconditions are read conditions other than one of standard readconditions and said auxiliary read conditions.
 6. The method of claim 1,wherein the plurality of read conditions use the same bias conditions,but differing reference points.
 7. The method of claim 6, wherein saidreference points are current levels.
 8. The method of claim 6, whereinsaid reference points are voltage levels.
 9. The method of claim 6,wherein said reference points are time values.
 10. The method of claim1, wherein the standard read conditions are a first set of biasconditions and the plurality of auxiliary read conditions are aplurality of auxiliary sets of bias conditions.
 11. The method of claim10, wherein the first set of bias conditions and the auxiliary sets ofbias conditions are distinguished from each other by a differing controlgate voltage.
 12. The method of claim 1, wherein said memory cells arenon-volatile memory cells.
 13. The method of claim 12, wherein saidmemory cells are charge storing devices.
 14. The method of claim 13,wherein said memory device is a flash memory.
 15. The method of claim 1,wherein said memory device includes a memory containing the memory cellsand a controller, wherein the extracting information on the distributionof programmed state populations of the memory cells and the determiningmodified read conditions are performed within the memory.
 16. The methodof claim 1, wherein said memory device includes a memory containing thememory cells and a controller, wherein the extracting information on thedistribution of programmed state populations of the memory cells and thedetermining modified read conditions are performed by the controller.17. The method of claim 1, wherein said standard read conditions areestablished by one or more reference cells.